1. Field of the Invention
The present invention relates to a light source unit which employs light emitting diodes or solid state light emitting devices and a projector which includes the light source unit.
2. Description of the Related Art
In these days, data projectors are used in many cases as an image projection apparatus for projecting screens and video images of a personal computer and, further, images based on image data stored in a memory card on to a screen. In projectors of this type, light emitted from a light source is concentrated on to a micromirror display device referred to as a DMD or a liquid crystal panel so as to display a color image on toga screen.
In these projectors, conventionally, projectors employing highly bright discharge lamps as light sources have been the mainstream of projectors. In recent years, however, there have been developments in projectors employing, as light sources, red, green and blue light emitting diodes or solid-state light emitting devices such as organic electroluminescence light emitting devices, and there have been made many proposals.
Compared with discharge lamps, light emitting diodes are advantageous in that they consume less power and highly reliable. However, the amount of light emitted by a single light emitting diode is still not good enough for use as a light source for a projector, resulting in a problem that a desired luminance cannot be obtained. Then, in order for light emitting diodes to be used as a light source of a projector, there exists a case where a plurality of light emitting diodes are disposed, so that lights from the plurality of light emitting diodes so disposed are combined for use.
For example, Japanese Unexamined Patent Publication No. 2005-274836 proposes a light source unit in which a plurality of white light emitting diodes are installed adjacent to one another on a substantially ellipsoidal or paraboloidal base member and are arranged so that their focal points are situated in the vicinity of a light incident surface of a light guiding or smoothing unit.
In addition, Japanese Unexamined Patent Publication No. 2006-208894 proposes a light source unit in which three light sources in each of which a plurality of light emitting diodes are arranged in an array are disposed on three sides of a cross dichroic mirror, and a rod integrator having a vertically elongated truncated quadrangular pyramid shape is disposed on an optical axis of a beam of light emitted from the cross dichroic mirrors so as to combine light emitted from the plurality of bright points.
In an optical system of an image display device or the like, a spatial expanse in which radiant flux which is handled effectively exists can be expressed by area and solid angle, and a product thereof is referred to as Etendue. The Etendue is a value preserved in the optical system.
In the projector which employs the display device such as the DMD described above, assuming that the area of an illuminated region of the display device is S′ and the solid angle of incident light which can be taken in by the area S′ of the illuminated region is Ω′, the Etendue of the display device is expressed by S′×Ω′. In addition, assuming that the area of a light emitting region of the light source unit is S and the solid angle of emitted light is Ω, the Etendue of the light source unit is expressed by S×Ω.
Since Etendue is the value preserved in the optical system, when the value of the Etendue of the light source unit which is expressed by S×Ω is smaller than the value of the Etendue of the display device which is expressed by S′×Ω′, all the light emitted from the light source unit can be used, and hence, all the emitted light becomes effective light. However, when the value of the Etendue of the light source unit which is expressed by S×Ω is larger than the value of the Etendue of the display device which is expressed by S′×Ω′, there occurs a situation in which some of the light emitted form the light source unit cannot be used.
In the related-art projectors which employ light emitting diodes, since the amount of light emitted from a light emitting diode is small, a plurality of light emitting diodes need to be provided in such a manner that beams of light emitted from the plurality of light emitting diodes are collected for use. In the event that the plurality of diodes are used, however, since the light emitting area becomes wide and hence, the value of the Etendue of the light source unit becomes larger than the value of the Etendue of the light display device, the amount of light that cannot be used is increased, and hence, there has been caused a problem that the utilization efficiency of the light emitting diodes is reduced.
In addition, in light emitting diodes, the light emitting efficiency of a green light emitting diode is lower than those of a red light emitting diode and a blue light emitting diode, a larger number of green light emitting diodes need to be provided than red light emitting diodes and blue light emitting diodes. In this case, even though light emitting diodes are collected together densely, the area of light emitting areas increases as the number of light emitting diodes provided increases, and therefore, since the value of Etendue of green light emitting diodes is increased, the ratio of effective light is decreased, and after all, there has still existed a problem that the issue of insufficient quantity of usable light from the green light emitting diodes remains unsolved.